Extreme Heat Raises Climate Change Questions, Concerns

Jul 06, 2012

The recent heat wave baking much of the country has prompted many people to ask: Is this due to climate change?

"This is always the million-dollar question, but unfortunately, there's no definitive way to answer it," says Steve Vavrus, a senior scientist in the Nelson Institute Center for Climatic Research at the University of Wisconsin-Madison. "We've experienced extreme heat, drought, floods, wildfires and windstorms throughout history, so in a sense this is nothing unusual. We need time to assess whether this year's set of extreme weather events falls outside of normal variations."

The list of unusual conditions over the past year is long. According to the National Oceanic and Atmospheric Administration (NOAA), last winter was the fourth-warmest on record in the United States; spring was the warmest since recordkeeping began in 1895; and April marked the end of the warmest 12-month period in U.S. history.

Still, Vavrus says scientists need more information to determine whether global warming is to blame. But he says heat waves like the current one will become more common on a warmer planet as we continue to add greenhouse gases such as carbon dioxide to the atmosphere, primarily through the burning of fossil fuels.

"I think it's a harbinger of what's to come under greenhouse warming," says Vavrus. "Virtually all climate models simulate more intense and frequent heat waves as the climate warms, and most of the world has experienced increases in extreme heat during the past several decades."

That's not good news for air quality or human health. Tracey Holloway, an associate professor of environmental studies, atmospheric and oceanic sciences, and civil and environmental engineering, says hotter temperatures lead to more ground-level ozone. Breathing ozone can damage lungs and worsen bronchitis, emphysema and asthma, and particulate air pollutants can affect a person's lungs and heart.

"Over the past few weeks, both ozone and particulate matter have increased across much of the country, with the Air Quality Index registering levels deemed 'unhealthy' or 'unhealthy for sensitive groups,'" says Holloway. "In the last week of June, moderate or unhealthy air covered a third to half of the continental U.S."

She says overall air quality typically degrades in hot conditions through a variety of physical and chemical processes.

"Often on the hottest days, we have very stagnant air, so emissions from cars and trucks, power plants and industry, along with natural emissions, have no place to go," she explains. "They hang around near the surface and react in the atmosphere to form ozone and some types of particles."

Holloway says heat-related events such as wildfires add harmful pollutants, and the increasing demand for air conditioning drives up power plant emissions.

In addition to causing respiratory issues, the excess heat poses direct health risks, according to Jonathan Patz, a professor of environmental studies and population health sciences and director of the UW-Madison Global Health Institute.

"Of all natural disasters in the U.S., heat waves have caused the most mortality," he says. "This week's heat wave began relatively early in the summer, and early-season heat waves can be particularly deadly, since we are less adapted to the heat, both physiologically and behaviorally."

Patz participated in a recent study of hospitalizations during heat waves in Milwaukee over a 16-year period.

"We found that risks to people with endocrine diseases such as diabetes, and renal disorders like kidney stones, increase during extreme heat waves," says Patz. "So do attempted suicides."

He says people in certain urban locations are more affected by the heat.

"Individuals who live on top-floor apartments, especially those without air conditioning, are at greater risk," he says. "Locations with more black asphalt roofs and roadways can be hotter than areas with more trees and green-space."

Patz says the health risks are compounded when extreme heat causes or combines with power outages, such as the extensive loss of electricity following intense wind storms that cut across the Midwest and mid-Atlantic last Friday.

"Such events are stark reminders of just how vulnerable we remain to extreme weather events," he says.

Whether these recent risks and impacts are the result of global climate change remains an unresolved question, according to Jack Williams, director of the Nelson Institute Center for Climatic Research. But he says there's no doubt that the global climate is changing.

"For the last 40 years of global warming, there is nothing comparable in the instrumental record since about 1880," Williams says. "To find comparable analogs for the amount of warming expected for this century under standard greenhouse gas emission scenarios, you have to go back to the climate changes accompanying the last deglaciation, about 20,000 years ago."